1. Field of the Invention
This invention is directed to a device employing the principle of gamma ray attenuation for determining the presence and location of decay in wooden poles, such as wood utility poles.
2. Description of the Related Art
Wood utility poles are still one of the preferred structures for supporting telephone and electrical transmission wires, and are selected to withstand a horizontal force applied two feet from the top of the pole when supported at the normal groundline. It has been found that this type of horizontal load produces a moment on the wood pole that results in the stress being concentrated at or near groundline. In the case of a 35 foot pole with a set of 6 feet the relative stress over the bottom 5 feet is shown as follows:
______________________________________ Distance Above Groundline (Ft) Relative Stress ______________________________________ 0.0 1.00 0.5 0.98 1.0 0.96 1.5 0.94 2.0 0.93 2.5 0.91 3.0 0.89 3.5 0.87 4.0 0.85 4.5 0.83 5.0 0.81 ______________________________________
Minimum circumferential dimensions of poes at groundline are so determined such that all poles of the same class regardless of length will withstand the same horizontal force. The assumption made is that the maximum bending moment (stress) is at groundline. However, if sufficient decay takes place at a location other than groundline the stress at that location may exceed the modules of rupture (MOR) causing the wood pole to break. For example, if at 5 feet above groundline a wood pole is experiencing 50% decay, the stress concentration would be sufficient to cause failure at this location.
The mathematical expression for internal moments beyond the elastic limit (break or yield point) is: EQU M=S I/C
where
M=moment at yield point PA1 S=modulus of rupture (MOR) PA1 I/C=Section modulus (moment of inertia of a circular cross section/diameter)
The modulus of rupture is therefore an experimentally determined value and the formula is empirical. The bending moment applied to a wood pole as a result of a horizontal load causes one section to undergo compression the other tension. Therefore there is an equalization of stress over the cross section. The outer material may be expected to develop greater ultimate strength than in simple stress on account of the reinforcing action of the material nearer the centre that has not yet been overstrained. In the case of wood the ultimate strength in tension is approximately one half of the MOR of horizontal bending.
In addition to stress factors caused by load, wood poles in situ are exposed to a number of natural factors, such as weathering, insect infestation and fungus, which may cause decay or rot in the pole, and create a potential hazard in weakening of the pole, especially in the first 5 foot region of greatest load-bearing stress.
One source of decay is ground rot, and to combat this, wood utility poles are generally butt-treated with preservatives. However, such preservative treatment generally extends only about one foot above ground level, and therefore, the decay sources mentioned above, such as constant dampness, insect infestation and fungus may enter the pole above the level of preservative treatment, but still within the region of greatest load-bearing stress, and cause shell-rot around the outside of the pole or decay in the centre of the pole.
It would therefore greatly benefit power distribution design and reliability to be able to detect the presence of decay and determine if a wood pole is still capable of supporting the designed load or increased loads.
The traditional method to determine the presence of decay in wood utility poles is incremental boring, in which a number of taps are mechanically taken from different areas on a pole as samples of the structure. However, incremental boring is time consuming and necessarily inaccurate, especially in the case of either localized decay or decay occurring at the centre of the pole.
It has long been known that gamma rays (electromagnetic radiation of very short wave about 10.sup.-12 cm, and high penetrating power) weaken as they pass through mass. The measurement of gamma ray attenuation in order to determine the density of materials has been used for a number of years, a well known example of which is the development of the CAT scanner for medical diagnosis and research.
The same principle has been applied in field use, to determine the presence of rot or decay in trees and wooden poles, particularly because the path of gamma ray emissions is not affected by magnetic fields.
For example, in U.S. Pat. No. 4,283,629, a device for determining the existence of disease, such as red rot, in living trees is disclosed. The device, which weighs in excess of 50 lbs, can be clamped onto a tree trunk in order to provide a series of readings around the circumference of the trunk at each latitude, and these readings may be reproduced by computer graphics three dimensional image on a computer screen.
Similarly, in an article entitled "CAT Scanner Inspect Poles In Fields", in Transmission & Distribution magazine dated August 1989, a similar device has been adapted for use on wooden utility poles, the device requiring hook-up to a computer for the rapid calculation of the thousands of "ray-sums" yielded by rotation of the device around the pole circumference at each level, to reconstruct the pole image.